The present invention involves spring elements for slide valves, especially those adapted for use on pouring ladles for molten metal, such as steel. A well known type of slide valve comprises a stationary refractory plate, fixed with respect to the ladle and a power operated sliding refractory plate movable with respect to the stationary plate. A pouring bushing may be provided on the downstream side of the sliding plate. Flow passages are provided in the respective plates and the pouring bushing to effect flow of metal from the ladle when the same are disposed in mutual alignment and to terminate flow thereof when the slide plate is moved to dispose the passages out of mutual alignment. Spring elements are provided in such valves to urge the facing surfaces of the respective plates into relatively tight mutual engagement.
Slide valves of this design have a satisfactory operating life. The flow passages in the respective plates which are subjected to the erosive effects of flowing metal and high temperatures are fabricated of mechanically strong and fire resistant material. Moreover, each of the plates can be easily replaced when damaged or when the wear limit is reached. Experience has shown that to ensure safe operation of such valves and to increase the operating life of the respective refractory plates and the pour bushing, it is important that the respective members be tightly pressed against one another by spring elements. The latter provide, on the one hand, the high thrust which is necessary to counteract the hydrostatic pressure acting on the slide plate, especially when the ladle is full and, on the other hand, limits to the excessive pressures that may occur during operation when the facing surfaces of the head and sliding plates are not perfectly even.
To obtain high thrusts and sufficient displacement in a small space, compact spring elements must be used. In conventional apparatus helical compression springs are employed which engage the inside surface of bolt heads, the outside surface of which supports or applies upward pressure to the sliding plate or to the pouring bushing. With this type of spring element the required pressure characteristics can be obtained and, if the displacement of the bolts is limited, the replacement of the sliding plate can readily be performed. Experience has shown, however, that the initially satisfactory characteristics of the compression springs are altered under the influence of heat to which the springs are subjected and after prolonged operation at high temperatures the thrust force exerted on the sliding plate is reduced. Therefore, in order to ensure safe operation, it is necessary to monitor the spring elements and to replace them after relatively brief periods of operation.
The high thermal stresses imposed on the spring elements can be alleviated by proper selection of high temperature resistant spring steels, by additional cooling and by reducing the amount of spring surface in contact with the heated members. While such measures may increase the operating life of the spring elements, their effective operating life is nonetheless unduly limited.
The use of gas operated spring elements in compression spring applications is known. Such spring elements have been described which are conventional pressure containing cylinders connected to storage tanks, for spring action applications. The pressure cylinders and especially their seals are inadequate for use at elevated temperatures. Moreover, the high pressure piping required for their use is bulky, and complicates maintenance inspection and replacement of parts. Also, although the use of storage tanks make it possible to obtain the desired flat spring characteristics, such use complicates installation and, in large measure, impedes cooling.
Moreover, expandable bellows spring elements have been considered for use in slide valve mechanisms for metal pouring vessels. U.S. Pat. No. 3,480,186, issued Nov. 25, 1969 to J. A. Grosko describes spring elements of this type wherein the operating fluid is continually supplied from an external source. While being capable of alleviating some of the problems referred to hereinabove, such spring elements are not totally dispositive of the problem in that the elements must be mounted in a fixed member of the mechanism and thereby lack flexibility of application. The arrangement is further deficient for the reason that each spring element is not mutually independent as manifest by the fact that the development of a leak in one element necessitates shutdown of the valve.
It is to the improvement of spring elements of the described type, therefore, that the present invention is directed.